x86/mm: Set old_guest_table when destroying vcpu pagetables

x86/mm: Set old_guest_table when destroying vcpu pagetables

Changeset 6c4efc1eba ("x86/mm: Don't drop a type ref unless you held a
ref to begin with"), part of XSA-299, changed the calling discipline
of put_page_type() such that if put_page_type() returned -ERESTART
(indicating a partially de-validated page), subsequent calls to
put_page_type() must be called with PTF_partial_set.  If called on a
partially de-validated page but without PTF_partial_set, Xen will
BUG(), because to do otherwise would risk opening up the kind of
privilege escalation bug described in XSA-299.

One place this was missed was in vcpu_destroy_pagetables().
put_page_and_type_preemptible() is called, but on -ERESTART, the
entire operation is simply restarted, causing put_page_type() to be
called on a partially de-validated page without PTF_partial_set.  The
result was that if such an operation were interrupted, Xen would hit a
BUG().

Fix this by having vcpu_destroy_pagetables() consistently pass off
interrupted de-validations to put_old_page_type():
- Unconditionally clear references to the page, even if
  put_page_and_type failed
- Set old_guest_table and old_guest_table_partial appropriately

While here, do some refactoring:

 - Move clearing of arch.cr3 to the top of the function

 - Now that clearing is unconditional, move the unmap to the same
   conditional as the l4tab mapping.  This also allows us to reduce
   the scope of the l4tab variable.

 - Avoid code duplication by looping to drop references on
   guest_table_user

This is part of XSA-310.

Reported-by: Sarah Newman <srn@prgmr.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>